After a technical inspection of an industrial plant, a power unit or the like, supplemental visual checks must be made for surface damage which could result in restricted operation or even complete breakdown or disintegration of the items to be checked. It is common practice to make such checks by the use of technoscopes comprising television or video cameras for aimed visual inspection. It is also customery to use photographic cameras for documenting discovered damage, for example changes attributable to material fatigue. Such checking operations are important because even the smallest damage to the surface may result in total functional failure or total destruction of the part concerned.
Such checks may be made by means of the method described in DE-A-3512602 by means of which the extent of the damage is measured directly or indirectly by means of a measuring grid or the like within the eyepiece of the optical system, once a crack or a damaged area has bee found. Substances reflecting ultraviolet light, such as magnetic powder, or fluids which allow visual determination to be made of the longitudinal or areal extent of crack damage may be used in assessing hair cracks, for example.
A complete evaluation of the damage for an expert opinion, cannot however, be made unless the depth of the damage as well as the area thereof is precisely determined.